Speed and precise regulation of synaptic transmission are critical for complex brain functions such as cognition and learning. Release of neurotransmitters from a presynaptic nerve terminal is often impaired in neurological disorders, including autism, schizophrenia, addiction and neurodegeneration. Exact knowledge of the molecular mechanisms for neurotransmitter release is thus critical for understanding brain disease. The active zone of a presynaptic nerve terminal is the site of neurotransmitter release. An active zone consists of a highly specialized network of proteins that organizes synaptic vesicles for fast Ca2+-triggering of release, a central requirement for speed and precision of synaptic transmission. It is our over-arching goal to understand how the protein machinery at the active zone operates. We approach this goal by dissecting the molecular functions of active zone components. ELKS proteins are highly enriched at active zones, indicating that ELKS functions in neuronal exocytosis at the active zone. Before release, active zones dock and prime synaptic vesicles for exocytosis close to presynaptic Ca2+-channels. How ELKS operates during these processes to control release is not understood, maybe in part because no systematic genetic approach has been taken in vertebrates to address ELKS function. We have now generated conditional knockout mice for both mammalian ELKS genes, ELKS1 and ELKS2. Ample preliminary data lead to our central hypothesis: ELKS proteins increase release probability though controlling presynaptic Ca2+-influx, and they modulate the size of the pool of readily releasable vesicles. We address separate components of this hypothesis in three specific aims, and we dissect the underlying molecular mechanisms. In aim 1, we hypothesize that ELKS1 and ELKS2 proteins have both shared and distinct functions. We determine how each ELKS gene contributes to the functions of active zones in neurotransmitter release by systematically studying presynaptic phenotypes in the newly generated conditional single knockout mice for ELKS1 and ELKS2, and in the ELKS1/2 double knockout mice. In preliminary experiments we find that ELKS proteins enhance presynaptic Ca2+-influx, and that individual and double ELKS deletions differentially affect the pool of readily releasable vesicles. In aim 2, we determine the mechanisms by which ELKS controls presynaptic Ca2+-influx. In aim 3, we propose a specific hypothesis that unifies effects on vesicle pools observed in ELKS mutants. We examine this hypothesis, determine the underlying molecular mechanisms and consider numerous alternative explanations. Our research is innovative because it addresses a novel hypothesis by a combination of genetic, biochemical and functional experiments of unique depth. Ultimately, this approach will lead to precise insights into the molecular control of neurotransmitter release, a key neuronal process that fails during various brain diseases.